Either Ntldr.exe is really missing or your boot.ini has got messed up and isn't pointing to it anymore. Try booting off the installation CD and selecting the option to repair an existing installation. Alternatively you could boot your system from a bootable CD such as a BartPE compilation or Knoppix and try to repair the problem (see above). Third option would be to stick your hard drive into another system that can read NTFS and try to repair the problem